Frequently reactive and chemically unstable, epoxide metabolites, formed primarily via the action of the cytochrome P450 monooxygenases, have been identified as ultimate carcinogenic and cytotoxic reaction products. Ultimately, the overall balance between bioactivation and detoxication pathways will determine the kinetics and fate of reactive intermediates within target cells. It appears likely that interindividual and tissue differences in susceptibility to toxic sequelae (such as cancers and certain birth defects) may be associated with an altered predisposition to detoxify epoxides. Although the cell has developed the capacity to metabolize epoxides through several pathways, e.g., via the glutathione S-transferases, the focus of this research program is to identify and characterize molecular aspects regulating the expression of a key enzyme involved in epoxide disposition, microsomal epoxide hydrolase (mEH). The mEH enzyme (EC 3.3.2.3) catalyzes the trans-addition of water to a broad range of epoxides and arene oxides, including oxides of the carcinogenic polyaromatic hydrocarbons. Our CENTRAL HYPOTHESIS is that altered expression and genetic variation within the mEH gene locus is a contributing factor toward differential cell and tissue susceptibility and individual risk of toxicity from chemical exposures. To test this hypothesis, two major specific aims are proposed. The first is to determine the mechanisms of post-transcriptional regulation of wild type and genetically variant mEH gene products in stably transfected cell lines representing various tissue origins. The second is to characterize the structural details of alternative mEH gene promoters with respect to DNA sequence composition, cell and tissue usage and interaction with nuclear regulatory proteins. Through analysis of the details that regulate expression of human mEH, genetic diversity among these pathways, and functional effects of altered mEH profiles within the cell, we hope to eventually develop biomonitoring, diagnostic, and perhaps, therapeutic approaches that will enable effective identification and intervention strategies for individuals at risk for toxic outcomes subsequent to toxic chemical exposures.